Composite structures are used in a wide variety of applications due to their high strength-to-weight ratio, corrosion resistance, and other favorable properties. In aircraft construction, composites are used in increasing quantities to form the fuselage, wings, horizontal and vertical stabilizer, and other components. For example, the horizontal stabilizer of an aircraft may be formed of composite skin panels co-bonded or co-cured to internal composite structures such as composite stiffeners or spars. The composite spars may extend from the root to the tip of the horizontal stabilizer and may generally taper in thickness along a spanwise direction to improve the stiffness characteristics of the horizontal stabilizer and reduce weight.
Composite stiffeners or spars may be provided in a variety of cross-sectional shapes. For example, a composite spar or stiffener may be formed in an I-beam shape by bonding or curing together the vertical webs of two C-shaped composite channels in back-to-back arrangement. Each one of the C-shaped channels may have horizontal flanges extending outwardly from upper and lower ends of a web. Each horizontal flange may transition into the web at a radiused web-flange transition. When the C-shaped channels are joined back-to-back to form the I-beam shaped stiffener, the radiused web-flange transitions result in a lengthwise notch along the upper and lower ends of the I-beam stiffener. The lengthwise notches may be referred to as radius filler regions or noodle regions. To improve the strength, stiffness, and durability of a composite structure, radius filler regions may be filled with radius fillers or noodles formed of composite material.
Unfortunately, existing radius fillers suffer from several drawbacks that detract from their utility. For example, existing radius fillers may exhibit cracking due to residual stress that may occur during the manufacturing process such as during cool-down from curing. The residual stress may occur as a result of a thermal mismatch between the radius filler and the adjacent composite laminates surrounding the radius filler. In addition, certain radius fillers may result in sub-optimal pull-off strength at the bond between the stiffener and the skin panel under structural loading.
As can be seen, there exists a need in the art for a radius filler that minimizes cracking during the composite manufacturing process and which provides favorable pull-off strength and which can be manufactured in a timely and cost-effective manner